1) The Ah receptor (AhR) mediates most of the toxic and tumor promotional properties of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other Polycyclic Aromatic Hydrocarbons (PAH) in rodents. Humans are exposed to low levels of PAH, the long term health effect(s) of which are uncertain. To understand the significance of excessive AhR activation it is essential to understand its normal role in cellular function, which is the first long-term goal of this project. II) The toxicological significance of prolonged Ahr-induced activation of CYP1A1, 1B1, and 1A2 expression many also be an important aspect PAH toxicity. Known endogenous CYP substrates play regulatory roles in directing cell growth, differentiation, proliferation or maintaining normal cellular homeostasis. Increased metabolism of such molecules by increased CYP1 expression due to AhR signaling might disrupt normal cell signaling pathways. Identification of endogenous substrates for the CYP1 family, especially those with regulatory functions, is the second long-term goal of this project. I) The Ah receptor (AhR) mediates moist of the toxic and tumor promotional properties of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other Polycyclic Aromatic Hydrocarbons (PAH) in rodents. Humans are exposed to low levels of PAH, the long term health effect(s) of which are uncertain. To understand the significance of excessive AhR activation, it is essential to understand its normal role in cellular function, which is the first long-term goal of this project. II) The toxicological significance of prolonged AhR-induced activation of CYP1A1, 1B1, and 1A2 expression may also be an important aspect PAH toxicity. Known endogenous CYP substrates play regulatory roles in directing cell growth, differentiation, proliferation or maintaining normal cellular homeostasis. Increased metabolism of such molecules by increased CYP1 expression due to AhR signaling might disrupt normal cell signaling pathways. Identification of endogenous substrates for the CYP1 family, especially those with regulatory functions, is the second long-term goal of this project. Two Central yet converging Hypotheses will be tested: (1) There are significant levels of endogenous ligand(s) (or other endogenous regulators) for the AhR in cells, and CYP-mediated metabolism alters their regulatory effect on AhR-mediated activity. (2) Endogenous substrates for CYP1B1 have regulatory roles in cellular metabolism, and one or more of these substrates function as ligands or regulators for AhR Both hypotheses are supported by preliminary findings from our laboratories and those of others demonstrating that constitutive AhR- mediated transcriptional activity is suppressed by CYP1 expression. Three Specific Aims will test these hypotheses: 1) Determine the mechanism by which CYUP1B1 regulates AhR transcriptional activity; 2) Identify endogenous AhR ligands/regulators and characterize their mechanism of action; 3) Identify endogenous substrates for CYP1B1 that also function as endogenous ligands for the AhR via labeling with 18O2 and ESI-MS analysis. Collectively, these studies will establish experimental systems for detecting endogenous AhR ligands and CYP substrates, and lead to identification of these compounds in mammalian cells. Results from these aims will provide an improved mechanistic understanding of: the normal function of the AhR and CYO isozymes in cells; the role of CYPs in maintenance of normal human health and in human disease processes; as well as health risks associated with exposure to PAHs.